1. Field of the Invention
The present invention relates to a dubbing system, and more particularly to a dubbing system for dubbing a digital information code train whose information has been compressed by highly efficient coding and to which an error correction code has been added.
2. Related Background Art
Attention has been recently paid to digital VTRs and digital audio tape recorders (DAT), having no deterioration during dubbing.
A common structure of a dubbing system using two digital VTRs is shown in FIG. 1. In this figure, S represents a slave VTR (on the recording side), and M represents a master VTR (on the reproducing side). For the purpose of simplicity, only the reproducing system of the master VTR (M) and the recording system of the slave VTR (S) are shown in FIG. 1.
In the master VTR (M), a recorded signal of a magnetic tape T1 is converted into an electrical signal by a magnetic head 26, and amplified by a reproducing amplifier 28.
An output of the reproducing amplifier 28 is demodulated by a digital demodulator (not shown), and thereafter applied to an ECC decoder 30. The ECC decoder 30 corrects a code error generated during reproducing, by using an error correction code contained in the recorded information code train. An output of the ECC decoder 30 is supplied to an expansion circuit 32 whereat it is subjected to a process opposite to that at the time of encoding, i.e., bandwidth expansion and decoding. The output of the ECC decoder is an information code (image code) bandwidth-compressed and encoded. An output of the expansion circuit 32 is supplied to a concealment circuit 34 which conceals a reproduced code error beyond the correction capability of the ECC decoder 30, by means of a process such as interpolation using image correlation. A D/A converter 36 converts an output of the concealment circuit 34 into an analog signal which is then outputted from an output terminal 38. An output of the concealment circuit 34 is supplied to a dubbing output terminal 35.
In the slave VTR (S), a video signal inputted to an input terminal 10 is converted by an A/D converter into a digital signal which is supplied via an N contact of a switch 14 to a bandwidth compression encoding circuit 16. An ECC encoder 18 generates an error correction code for correcting a code error generated at the time of reproducing, and adds a so-called check bit to the bandwidth-compressed and encoded signal from the bandwidth compression encoding circuit 16. An output of the ECC encoder 18 is modulated by a digital modulator (not shown), and thereafter amplified by a recording amplifier 20 to be magnetically recorded on a magnetic tape T2 by a magnetic head 22.
During dubbing, an output of the concealment circuit 34 of the master VTR (M) is inputted via the terminal 35 to the dubbing input terminal 37 of the slave VTR (S). This connection is made so as to dub an error code data when a code error that can not corrected is generated in a reproduced information code data. The concealed information code train is therefore inputted to the terminal 37. The concealed code data is applied to the compression encoding circuit 16 to be subjected again to the compression encoding process, and thereafter magnetically recorded on the magnetic tape T2.
A digital VTR is not influenced by distortions on the electromagnetic conversion system, contrary to the case of an analog VTR. Therefore, image deterioration is caused only by bandwidth compression and error concealment, the latter being executed when a number of errors are generated. Further, deterioration caused by distortions on the electromagnetic conversion system of a digital VTR will not be accumulated during dubbing, which is advantageous over an analog VTR.
However, in the above-described conventional system, there is a problem of image deterioration caused by repetitive compressions and expansions if dubbing is repeated many times, because a concealed output from the concealment circuit 34 is supplied to the D contact of the switch 14 and compressed again to magnetically record it.